The present invention relates to the formation of contact bumps or columns for infrared radiation detector arrays.
Such arrays of bumps are formed on a substrate. The substrate may be a wafer of silicon integrated circuits, or a wafer of infrared detector arrays, or other material.
In the prior art, an organic positive photoresist is applied to the surface of the substrate. A mask is applied to the surface of the photoresist, which mask has openings where the columns are to be formed. These portions of the photoresist are exposed to UV light through the openings in the mask. The photoresist is then developed, which removes the positive photoresist that was exposed to the light through the mask, forming openings through the photoresist to the substrate surface where the contact bumps are to be formed.
The contact material, such as indium, is then deposited. The entire wafer surface is coated with the metal, filling the holes in the photoresist and coating the surface of the remaining photoresist. The metal is deposited by electroplating, E-beam deposition, evaporation, or sputtering, all of which are known in the art.
The next step is to dissolve the photoresist from under the indium, so the indium layer can be removed, leaving only the indium bumps on the substrate surface. To be able to dissolve the photoresist, the solvent must have access to the photoresist, which requires that openings be present through the indium.
The most likely place for such openings to be formed are the corners at the edges of the holes in the photoresist. If the corners at the edges of the photoresist are sharp, as they would be if the edges of the photoresist were vertical, such as shown in FIG. 1, a little ultrasonic agitation could break through the small amount of indium at the corners 13 of the photoresist 15, providing the necessary access. Unfortunately, attempts to control the sidewall slope by exposure, contrasts enhancement material, developer concentration, and clorobenzene soak have not been successful for thick positive photoresist of 5-50 microns in thickness. In actual practice, the holes in the photoresist tend to have gently sloped edges, providing areas for the detector material to adhere in relatively thick layers and reducing the sharp corners that are likely to provide places for holes to develop in the detector material. An example is shown in FIG. 2. Thus, extensive ultrasonic agitation has been required to form openings through the indium "to provide access for the solvent to the photoresist 14."
Another method used to form the conductive pads or bumps is to initially coat the substrate surface with the conductive material (indium). Positive photoresist is then applied on top of the indium layer to define the future pads. The unwanted indium is etched away, leaving only the pads desired. This method requires careful control of the etch, which in turn requires tight control of the etching agent. Consequently, formation of uniform bumps or pads is extremely difficult.